One who slanders another behind his back, i.e. in his absence. His bosom friends are become his back biters, said of a lousy man.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
babble * backedNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes